High pressure sodium discharge lamps with saturated sodium/mercury amalgam fills are known to the art. These lamps are overdosed so that a liquid amalgam pool remains in the lamp during operation and the sodium and mercury pressures in the arc are regulated by the temperature of the coldest spot in the arc tube. Current lamp design prescribes the use of very highly overdosed amalgam pills. During the lamp life the lamp voltage of such lamps will slowly rise and eventually lead to extinction when the lamp voltage exceeds the available main voltage. Two reasons for this voltage rise can be identified.
Firstly, the highly overdosed pills supply ample sodium in the arc tube so that the loss of sodium from the arc due to chemical reactions can be compensated. However, this compensation is only partial, since as the sodium fraction in the liquid decreases, the mercury to sodium ratio in the vapor rises. Since mercury serves as a buffer gas to raise the lamp voltage, the latter effect will induce lamp voltage rise together with sodium pressure drop.
Secondly, emitter material is lost from the electrodes due to evaporation and sputtering. This leads to less efficient and hotter electrodes and to blackening of the arc tube wall. Both these effects cause the coldest spot temperature to rise. Consequently, the vapor pressure of mercury and sodium above the amalgam will increase, leading again to lamp voltage rise.
A second disadvantage of conventionally overdosed lamps is the lamp voltage instability with input voltage and fixture temperature since both change the coldest spot temperature of the ar tube.
Both voltage instabilities (temporal and thermal) can be limited using unsaturated dosage of the arc tubes. In these lamps the amalgam is completely evaporated during operation so that the gas density becomes independent of the coldest spot temperature and this assures a more stable voltage. Since sodium is highly reactive at the temperatures prevailing in a high pressure sodium lamp, an unsaturated vapor lamp always shows a drop in sodium density, and consequently lamp voltage, during the lamp life. To assure a sufficient sodium density and lamp voltage at the end of the rated life, an unsaturated vapor lamp initially operates at a higher voltage than rated and often at a higher sodium density in the arc than desired for maximum luminous efficiency. The decreasing sodium pressure entails changing luminous flux and color characteristics. The decreasing voltage leads to power and/or current changes according to the ballast on which the lamp is operated. The current technology allows to produce unsaturated vapor type high pressure sodium lamps with sufficiently long life only at rated wattages above 150W. These lamps do exhibit the above-mentioned disadvantages. In low wattage high pressure sodium lamps, the current state of the art can not maintain sufficiently high sodium pressures during the life of a saturated vapor lamp.
High pressure sodium lamps with sodium dosage such that 80 percent or more of the sodium is initially in the vapor state are described in European application 87/302218, which corresponds to U.S. Pat. No. 4,755,721. In these lamps the sodium content is not optimized in any way and the 20 percent or less excess sodium is not intended to compensate for sodium lost from the arc during a significant part of the lamp life. On the contrary, said lamps are described to be a variety of the unsaturated vapor type since they becomes unsaturated fairly early in life.